1. Field of the Invention
The present invention relates to an ionic conductive polymer electrolyte and an electrochemical device such as an electric double layer capacitor, an electrolytic capacitor and a lithium secondary battery produced by using the ionic conductive polymer electrolyte.
2. Description of the Related Art
Various kinds of electrolytes are used for driving an electrochemical device such as an electric double layer capacitor, an electrolytic capacitor and a lithium secondary battery. Each of these electrolytes is, for example, a combination of a medium with a high dielectric constant and a salt which can produce ions. Such electrolytes are required a high voltage to be decomposed, and are required to have a high electric conductivity, a high boiling point, a low freezing point and a low vapor pressure. Although a number of liquid electrolytes are used at present, most of those having a high boiling point generally have a high viscosity, and therefore, the electric conductivity thereof is low. When a liquid electrolyte with a low boiling point is used so as to attain a high electric conductivity, such an electrolyte is likely to evaporate while used for a long period of time.
An electrolyte conventionally used for driving an electrolytic capacitor is obtained, for example, by dissolving an organic ammonium salt in an organic solvent such as ethylene glycol and .gamma.-butyrolactone. Electrolytes for driving a lithium secondary battery and an electric double layer capacitor are obtained, for example, by respectively dissolving an inorganic lithium salt and an organic ammonium salt in solvents with a high dielectric constant such as propylene carbonate and the like. These conventionally used electrolytes have a high electric conductivity.
However, electrochemical devices using the electrolyte made of an organic solvent have a difficulty in maintaining their reliability for a long period of time due to the evaporation of the organic solvent because an organic solvent with a low viscosity such as ethylene glycol has a low boiling point as mentioned above. Further disadvantageously, the electric conductivity of such an electrolyte is largely decreased as the temperature is lowered. Electrolytes using a solvent with a high dielectric constant such as .gamma.-butyrolactone and propylene carbonate can exhibit a sufficiently high electric conductivity at room temperature. But these electrolytes inherently have a high viscosity, and thus, especially at a temperature below 0.degree. C., the electric conductivity thereof is largely degraded. Therefore, in an electric double layer capacitor, which uses an electrode formed of a mixture of such an electrolyte and activated carbon, an impedance is largely increased especially at a temperature below -10.degree. C.
An electrolyte is obtained by dissolving a lithium salt such as LiClO.sub.4, LiBF.sub.4 and LiPF.sub.6 in solvent with a high dielectric constant, and is used for driving a lithium secondary battery using CoO.sub.2 as a positive electrode active material and having a voltage of 4 V or more. In such a case, especially when such a lithium secondary battery is charged and stored at a temperature of about 70.degree. C., the electrolyte is decomposed with time to generate a gas, resulting in deteriorating the characteristics of the lithium secondary battery.
Various solid electrolytes have recently been developed in order to solve the above described problems. A solid electrolyte used to be regarded unsuitable for practical use because ions are difficult to move in a solid as compared with in a liquid, and thus, the ionic conductivity of a solid electrolyte is markedly lower than that of a liquid electrolyte. However, a number of solids have recently been discovered which can selectively transmit only specific kinds of ions and have a high electric conductivity. When such a solid electrolyte is used in an electrochemical device, there is no possibility of leakage of the electrolyte can be minimized. As a result, the structure of the electrochemical device can be simplified, resulting in a compact device with a long life. Moreover, such a device can be used at a higher temperature, resulting in an electrochemical device with a higher quality. As such a solid electrolyte, combinations of various organic or inorganic solids-and various salts are now proposed. Especially, a solid electrolyte using an organic polymer, and an electrochemical device using the solid electrolyte are proposed.
An electric double layer capacitor is a rechargeable power supply to be used similarly to a secondary battery. The electric double layer capacitor generally has a laminate structure having a current collector, an electrode, an electrolytic layer, another electrode and another current collector. The current collector herein is a electrically conductive plate made of an electrically conductive material such as aluminum. In a conventional electric double layer capacitor using a liquid electrolyte, an electrode is produced by applying a paste including activated carbon, a binder resin such as fluororesin and an electrically conductive material such as acetylene black onto the current collector and heat-treating the resultant current collector with the paste. The electrolytic layer is produced by soaking a separator made of a cloth or the like with a liquid electrolyte.
The liquid electrolyte to be used in the electric double layer capacitor generally contains, for example, sulfuric acid or an organic solvent. The liquid electrolyte which contains sulfuric acid has a withstand voltage as low as approximately 1.2 V, which is lower than the voltage required for electrolysis of water. However, this liquid electrolyte has an extremely high electric conductivity of approximately 0.7 S/cm. Due to these characteristics, such a liquid electrolyte containing sulfuric acid is used for a power supply where a comparatively large output current is required, for example, for a backup power supply for driving an electric appliance whose power supply is cut off.
An example of the liquid electrolyte which contains an organic solvent includes, for example, an electrolyte containing propylene carbonate as a solvent and tetraethylammonium perchlorate as a solute. An electric double layer capacitor which employs this liquid electrolyte has a withstand voltage of 2.4 V, which is twice as high as that of the conventional electric double layer capacitors which employ the liquid electrolyte containing sulfuric acid. However, the electric conductivity of this electrolyte is about 0.01 S/cm, which is lower than that of the liquid electrolyte containing sulfuric acid by about two orders of magnitude. Due to these characteristics, such a liquid electrolyte containing an organic solvent is used, for example, as a power supply for a backup memory in a compact electronic appliance (Ichiro Tanahashi et el., Carbon, Vol. 29, No. 7, p. 1033, 1991).
However, these liquid electrolytes can leak from such a capacitor as mentioned above. Therefore, a capacitor utilizing a solid polymer electrolyte which has substantially no possibility of leakage is now proposed.
Examples of the uses of the solid polymer electrolyte include an electric double layer capacitor which uses, as an electrolyte, a mixture of polyvinyl alcohol and a lithium salt such as lithium perchlorate, etc. in polyvinyl alcohol and has an electrode produced by soaking porous carbon with the electrolyte (J. Power Sources, 36, p. 87, 1991); and an electric double layer capacitor which uses, as an electrolyte, a mixture of cross-linked polyethylene oxide and an alkaline metal salt such as lithium perchlorate and has an electrode produced by mixing the electrolyte with activated carbon (Japanese Laid-Open Patent Publication No. 2-39513). However, when an electrochemical device is thus produced by using an electrolyte including an alkaline metal salt such as a lithium salt, it is necessary to completely remove moisture from the other structural materials to be used. It is especially difficult to completely remove moisture from porous carbon, that is, a material for an electrode, and to fabricate an electrochemical device while keeping the porous carbon in a completely dry state. Therefore, such a solid polymer electrolyte has not yet been practically fabricated or used.
Moreover, such a solid electrolyte has a problem of deterioration due to ambient moisture. A thin device can be produced generally by using a solid electrolyte. Such a thin device is generally sealed with a metal foil such as an aluminum foil whose surface is coated with an insulating sheet made of polypropylene, etc. When the above-mentioned electrolyte using a lithium salt is sealed in this manner and the obtained device is allowed to stand, for example, at a temperature of 60.degree. C. and a humidity of 90%, the performance of the device starts to degrade after about 2 months. This appears to be due to moisture gradually entering the inside of the device through the sealing material.
In order to solve this problem, the use of an ammonium salt, which is the same kind of ammonium salt that is used in the above-mentioned liquid electrolyte made of an organic solvent, has been taken into consideration. It is, however, known that an ionic conductive polymer electrolyte in which an ammonium salt has been dissolved generally has an extremely low ionic conductivity (S. Chandra et al., Solid State Ionics, 40/41 (1990) p. 651). The inverse number of the ionic conductivity of an electrolyte used for constituting a capacitor is in proportion to the electric resistivity of the capacitor. Therefore, when the ionic conductivity of the electrolyte is too small, the electric resistivity of the capacitor is too large to output a sufficiently large current. As a result, such a capacitor can not be practically used.
A salt used in an electrolyte for an electric double layer capacitor needs to have an ionic radius as small as possible because the ions of such a salt are required to enter micropores with a size of about several tens angstroms in porous carbon such as activated carbon, that is, the material for the electrode. As is well known, however, it is difficult to dissolve a salt having an anion and a cation with a smaller ionic radius in a base polymer.
In order to obtain a polymer electrolyte with a high ionic conductivity by using an ammonium salt, a base polymer and an ammonium salt should be carefully selected, considering the foregoing factors of the ionic conductivity, the ionic radius, etc. But a specific electrolyte having practicable electric characteristics has not been developed as yet.
An attempt to use a solid polymer electrolyte in other electrochemical devices such as an electrolytic capacitor and a lithium secondary battery which is different from the electric double layer capacitor has been made.
The electrolytic capacitor is used as a device in an electric circuit, and generally comprises a positive electrode, a dielectric layer, an electrolytic layer and a negative electrode. The positive and negative electrodes are made of a metal such as aluminum and stainless steel. The dielectric layer is made of a metal oxide such as Al.sub.2 O.sub.3. The electrolytic layer is, as in the electric double layer capacitor, obtained by soaking a separator with a liquid electrolyte.
The lithium secondary battery is used as a rechargeable power supply, and generally has a laminate structure comprising a current collector, a positive electrode, an electrolytic layer, a negative electrode and another current collector. The positive electrode is obtained by applying a mixture of LiCoO.sub.2 as an active material in the positive electrode, a binder resin, and an electrically conductive material such as acetylene black on the current collector made of a metal and by heat-treating the resultant current collector. The electrolytic layer is obtained by soaking a separator with a liquid electrolyte. The negative electrode can be categorized within the following types: one produced from a metal; one produced by using lithium as a negative electrode active material in the same manner as in the positive electrode; and one produced from graphite.
Generally, such a laminate structure is provided with a terminal on its positive electrode or current collector, rolled up and put in a container to be used as an electric double layer capacitor, an electrolytic capacitor or a lithium secondary battery.
The electrochemical devices such as the electrolytic capacitor and the lithium secondary battery are also expected to attain a compact size and a long life by using the solid polymer electrolyte. However, a specific electrolyte for such electrochemical devices having practicable electric characteristics has not been proposed as yet.